Download Ray Davis: The Scientist and the Man

J. Bahcall/Nuclear Physics B (Proc. Suppl.) 48 (1996) 281–283
Ray Davis:
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The Scientist and the Man
John Bahcall
Institute for Advanced Study
Olden Lane, Princeton, NJ 08540
Ray Davis discovered solar neutrinos. Without
Ray, I believe this discovery would not have been
made in the 20th century. You can get some feeling for how hard solar neutrino experiments are
from the fact that it took more than twenty years
after Ray began his experiment before a second
experiment (Kamiokande) was started.
In the early days of the subject, Ray was the
only experimentalist willing to risk a significant
fraction of his research career in an effort to try
to detect solar neutrinos. Solar neutrinos was not
a fashionable research topic in the early 1960’s.
In fact, all the people working steadily on solar
neutrinos (theorists and experimentalists) could
(and often did) fit comfortably into the front seat
of Ray’s car.
A few words about Ray himself. Ray was born
on October 14, 1914. He will be 81 next month.
He married Anna Tomre in 1948, 47 years ago.
They have five children, and nine grandchildren,
in all of whom they delight and take great pride.
Ray’s Ph.D. is in Physical Chemistry from Yale in
1942. He was a member of the Chemistry Department at Brookhaven National Laboratory from
1948 to 1985, and is now emeritus. Since 1985, he
has been Research Professor of Astronomy at the
University of Pennsylvania. Despite his claims to
be a chemist, most of Ray’s research papers are
published in the Physical Review or the Physical
Review Letters.
Ray is best known for his work using chlorine as a detector of neutrinos, as first suggested
by Bruno Pontecorvo. In 1955, he published in
Phys. Rev. the results of his search for antineutrinos from a reactor. He showed that reactor anti-neutrinos are not absorbed by chlorine,
ν̄e + 37 Cl 6⇒ e− + 37 Ar, which was the first experimental evidence that an antineutrino is differ-
ent from a neutrino. In 1964, we published backto-back papers in Phys. Rev. Lett. arguing that
the expected rate of capture of solar neutrinos in a
chlorine detector was large enough to be detected
in a practical experiment. In 1968, the first results of the chlorine experiment were published;
the observed rate was less than the theoretical
prediction. The situation has not changed much,
as far as the chlorine experiment is concerned, in
the subsequent two-and-a-half decades.
For the first 20 years, Ray’s experiment was
the sole basis for the claim that solar neutrinos
had been discovered. It was also the sole basis
for the solar neutrino problem, the discrepancy
between Ray’s experiment (continually refined)
and the theoretical expectations (continually refined). In the past five years, three beautiful
experiments, Kamiokande, GALLEX and SAGE,
have confirmed the discovery of solar neutrinos
and provided further evidence for a discrepancy
between theory and observation.
Ray is widely admired as a great scientist. He
also had at least one great political triumph, a
fact which – until today – was known only to Ray
and myself. In order to tell you about his political
success, I have to tell you a bit about our joint
efforts.
Our collaboration began in 1961, when Ray
wrote to me (at Willy Fowler’s suggestion) to ask
if I could calculate a precise rate for the reaction
e− + 7 Be → νe + 7 Li in the sun. I did and, in
1962, I moved to CalTech, in part to carry out
the first detailed solar model calculation of neutrino fluxes (with stellar model experts, Iben and
Sears). Both Ray and I were enormously disappointed that the calculated rate was too low by
an order of magnitude to be detected in what Ray
believed he could achieve in a practical chlorine
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J. Bahcall/Nuclear Physics B (Proc. Suppl.) 48 (1996) 281–283
experiment. The situation changed in 1963, with
the realization that the superallowed transition
from the ground-state of 37 Cl to the isotopic analogue state at about 5 MeV in 37 Ar increased
the predicted capture rate by about a factor of
20. This led to our 1963 joint conference paper
and our pair of Phys. Rev. Lett. papers arguing that a chlorine solar neutrino experiment was
practical. It also led to Ray’s greatest political
achievement.
Ray invented a way to sell the experiment to
Maurice Goldhaber – then director of Brookhaven
National Laboratory (and therefore Ray’s boss).
Maurice’s attitude toward astrophysics was said
to be: “No astrophysicist can calculate anything
with sufficient precision to be of any interest to
any particle physicist.” Ray proposed, and I reluctantly agreed, that in our crucial meeting with
Goldhaber requesting Brookhaven support for the
chlorine experiment we would not discuss the astrophysical motivation. As Ray suggested, we discussed at the meeting only the nuclear physics
of the much-increased predicted capture rate via
the isotopic analogue state and the experimental
tests of this idea (which I had also worked out)
that were possible at Brookhaven. Both Maurice
and his wife are distinguished nuclear physicists
and Maurice (an insightful theorist and a talented experimentalist) loved, according to Ray,
to talk about any new idea in physics. As Ray
had hoped, Maurice was much interested in the
nuclear physics ideas and tests and, perhaps incidentally, also approved the solar neutrino experiment.
Because of the many tests of the chlorine experiment that Ray performed and because of Ray’s
character and personality, other scientists came
to believe that it was worthwhile to invest years
trying to observe solar neutrinos.
Before each extraction, Ray added a known
amount of carrier (non-radioactive) argon and
showed that he got the correct amount of carrier
argon out of his tank at the end of each run. He
placed a small neutron source near the center of
the tank and showed that this produced the expected number of 37 Ar atoms. At Willy Fowler’s
suggestion, Ray introduced 500 37 Ar atoms into
his detector, stirred them up, and showed that he
extracted the expected number of argon atoms.
He also performed an overall test of the experiment using 36 Cl, which beta-decays to 36 Ar in
a way that mimics well what occurs in neutrino
absorption.
Ray’s character and personal approach to the
experiment were as important in gaining credibility for the experiment as the actual tests he performed. Over the past 30 years, Ray and I have
appeared together more than 100 times in formal discussions (like today’s) of solar neutrinos,
and many more times informally. I have never
seen Ray loose his patience or get angry in answering questions about his experiment. He has
answered each question with friendly directness,
with complete openness, and – whenever possible – with empirical tests. He invited everyone
who was interested to come to Brookhaven, or
to the Homestake mine, to see him perform the
experiment and to discuss techniques. Many distinguished physicists did come and Ray explained
every detail to them.
Ray loves science. He wants to see good experiments done and he wants to understand the
results. He is not interested in personal credit.
Let me give just a few illustrative examples. Ray made the 7 Be target for Parker’s famous 7 Be(p, γ)8 B experiment and he provided
low background prototype 71 Ge counters for the
SAGE experiment. The chemical techniques used
in both the GALLEX and the SAGE experiments
were originally developed, in collaboration with
some colleagues (especially Cleveland, Dostrovsky, Lande, and Rowley), in Ray’s laboratory
and were made available to any interested scientist. Many of the senior people in this room
had their early lessons in low counting rate experiments and in background suppression under
Ray’s tutelage. Ray is generous and helpful to
other scientists in their work.
Before I summarize these remarks, I want to
show you two pictures taken in 1966 when Ray’s
tank was first being installed. Here is the first picture, Figure 1. Notice the look on Ray’s face. Can
you figure out what was bothering him? Anyone
care to guess? No. Well, I had just explained
to Ray (jokingly) that I had found an error of a
factor of 4π in my theoretical calculation.
J. Bahcall/Nuclear Physics B (Proc. Suppl.) 48 (1996) 281–283
Figure 1. Ray Davis (circa 1966) with a quizzical
look.
The second figure shows, from the same period,
Ray explaining some of the features of the tank
to a young assistant. (I hope that despite the
ravages of time, you can recognize both people in
Figure 2.)
Ray’s principal scientific achievement is the observation of solar neutrinos. The nuclear reactions that produce the neutrinos also cause the
sun to shine. Thus Ray has shown experimentally that nuclear fusion among light elements is
responsible for energy generation in the sun. This
closes experimentally the scientific debate on the
age of the earth and the energy generation of the
sun that originated with Helmoltz and Darwin
in the middle of the 19th century. His experiment also created the first solar neutrino problem, the discrepancy between his measured rate
and the standard model predictions. Together
with the other three great pioneering experiments
(Kamiokande, GALLEX and SAGE), the chlorine
experiment shows that we can take advantage of
the Solar Neutrino Opportunity – the possibility
of using solar neutrino beams to do unique particle physics experiments.
Equally important, in my view, Ray has shown
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Figure 2. Ray Davis (also circa 1966) with a
young assistant.
that it is possible to be a great scientist and also
a great man. He is kind, modest, friendly, open,
enthusiastic, and generous. As the chair of this
session (Till Kirsten) remarked to me last night,
Ray treats everyone with respect and generosity,
independent of whether the person to whom he
is talking is a student novice or a senior professor, a friendly miner or an unfriendly scientist. I
hope you will join me in expressing professional
admiration for Ray’s work and personal affection
for him as a wonderful individual.